1. Field of the Invention
This invention relates to accelerators and a process for their use in the layer-refining application of phosphate coatings to metal surfaces by means of phosphating solutions based on zinc phosphate and/or iron phosphate and/or zinc-iron phosphate as the principal layer-forming component.
2. Description of the Related Art
It has long been known that iron phosphate layers can be formed on iron and steel surfaces. Alkali and/or ammonium orthophosphate solutions having a pH of from 3.0 to 6.5 are used for this purpose ("non-layer-forming phosphating").
Processes by which zinc phosphate layers are formed on metal surfaces are also known ("layer-forming phosphating"). Layers such as these improve corrosion prevention and lacquer adhesion. Earlier processes required high reaction temperatures and a considerable treatment time for layer formation. The layer-forming process can be shortened by the addition of accelerators. Above all, oxidizing agents, such as nitrate, nitrite, chlorate, hydrogen peroxide and organic nitro compounds, play an important role as accelerators.
Thus, British Patent Application No. 2,074,611 and corresponding German Patent Application No. 30 16 576 describe a process for accelerating the formation of phosphate layers based on zinc phosphate, the solution applied containing nitrite and chlorate as accelerators. A process based on a solution of zinc phosphate uses a combination of chlorate and a water-soluble aromatic nitro compound, preferably Na-m-nitrobenzene sulfonate, as accelerator (see British Patent Application No. 2,102,839 and corresponding German Patent Application No. 32 24 923). A comparable combination is claimed in British Patent No. 1,542,222.
U.S. Pat. Nos. 4,292,096 and 4,419,147 as well as corresponding German Patent Application No. 30 04 927 also describe a process for forming phosphate layers on metal surfaces using zinc phosphate solutions containing nitrite and/or organic nitro compounds and, optionally, also chlorate.
Moreover, the use of water-soluble aromatic nitro compounds in accelerator systems for phosphating processes, during the reaction with the metal surface, leads to serious discoloration of the phosphating solutions and also to the formation of voluminous sludge. Both disadvantages make the process difficult to carry out and necessitate permanent "restrengthening", i.e. readjustment of the contents of the solutions.
In addition, it is known from U.S. Pat. No. 3,923,554 that comparatively thick phosphate layers can be formed on metal surfaces. These layers reduce frictional resistance during cold-forming. Phosphate coatings such as these weigh between 10.0 and 22.0 g per square meter. The formation of coatings such as these requires treatment times of several hours and treatment temperatures in the range from 90.degree. to 95.degree. C. In this case, formation of the coating is accelerated by nitrites. U.S. Pat. No. 3,923,554 describes a process in which layer formation is accelerated by the addition of up to 2 g/l of sodium nitrite. However, since nitrite concentrations as high as these in the solutions applied interfere with formation of the phosphate coating through passivation of the metal surfaces, excesses of nitrite are bound--according to U.S. Pat. No. 3,923,554--by means of urea, its adducts and also sulfamic acid, ascorbic acid or hydroxyl amine. These substances thus prevent the nitrite-induced passivation of the metal surface.
The nitrite content of the phosphating solution is generally adjusted to at most 0.1 g/l. In many cases, nitrite concentrations of this order in the treatment solution are sufficient to obtain the formation of phosphate coatings on metal surfaces. In addition, a number of factors, for example the temperature of the phosphating solution, the available oxygen, the reactivity of the metal surfaces to be treated, mechanical agitation of the phosphating solution, the spraying pressure and the pH-value, influence the effect of nitrite on the formation of the phosphate coating. It follows from this that, in the presence of nitrite, the performance of the bath depends upon a number of intricately interrelated factors.
Another factor to be taken into account is that phosphating solutions frequently contain nitrate. Carrying out the phosphating process at elevated temperature in the presence of nitrates as oxidizing agents leads increasingly to autoreduction of the nitrate with formation of more nitrite. The formation of this additional nitrite is difficult to control and undesirable, because, as mentioned above, passivation of the metal surfaces occurs to an increasing extent.
One particular disadvantage lies in the fact that the use of nitrite-containing systems for accelerating phosphating solutions leads to the release of physiologically harmful nitrous gases. This disadvantage makes it advisable to avoid using nitrite or even nitrate as phosphating accelerators or to carry out the reaction under such conditions that no nitrite is formed.
Adjustment and maintenance of the pH are crucially important to the formation of a good phosphate coating. The pH may be in the range from 1.8 to 5.8 and is preferably adjusted to the required level by means of phosphoric acid. However, sulfamic acid (see British Patent No. 1,360,266 or corresponding German Patent Application No. 21 52 446) and a combination of sulfamic acid and phosphoric acid have also been used for this purpose. Due to the lower acidity of the organic component, however, the concentrations required are distinctly higher (up to 9.5% by weight, based on the solution applied) than is the case where phosphoric acid alone is used.
Further disadvantages of the above processes are that the various weights per unit area in which the phosphate coating can be applied are difficult to control and that the phosphate coatings obtained are not sufficiently fine-grained for effective lacquer adhesion. In addition, it is not possible in the above processes to adjust specific coating weights and grain sizes by altering simple parameters or to control the formation of phosphate coatings as a function of temperature.
Thick and fully developed phosphate coatings with weights per unit area of from 10 to 35 g/m.sup.2 are required for corrosion prevention and for lubricant carriers in cold forming operations. Weights per unit area as high as these are normally obtained at phosphating bath temperatures of from 70.degree. to 100.degree. C. German Patent Application No. 22 41 798 describes one such nitrate-accelerated immersion process in which the ratio by weight of P.sub.2 O.sub.5 to Zn to NO.sub.3 has to be adjusted to 1:(0.7-2.0):(0.3-0.7). German Patent Application No. 15 21 927 also claims a nitrate-accelerated process in which the ratio by weight of P.sub.2 O.sub.5 to Zn to NO.sub.3 is disclosed as 1:(1.4-2.6):(2.0-4.3). In both processes, a small addition of sodium nitrite during preparation of the bath has to be made to "initiate" the phosphating solution. The continued formation of nitrite which is required for the formation of a phosphate coating on the metal surface takes place autocatalytically from nitrate. As a result, the iron (II) entering the bath during the throughput of iron and steel is in danger of being oxidized to a significant extent into iron (III), resulting in precipitation and undesirable sludge formation.
In practice, soaps in conjunction with phosphate layers are used as lubricants in cold forming. The zinc phosphate layers on the workpiece may be partly reacted with alkali soaps in such a way that particularly effective zinc soaps are formed. In this case, the tertiary zinc phosphate of the layer reacts with sodium soap to form zinc soap and tertiary sodium phosphate. For the reaction, the phosphated workpieces are immersed in a soap bath for 2 to 10 minutes at 70.degree. to 80.degree. C. The highest degree of reaction and therefore the best forming results are obtained with special reactive soap lubricants, and immersion baths mixed with quantities of from 2 to 10% by weight have a pH of from 8 to 10.
The formation of the phosphate coatings may be influenced by special prerinses. With prerinses of the type in question, it is frequently possible to eliminate the layer-degrading effects of preceding treatments, for example alkaline degreasing or pickling. Because of this, prerinses of the type in question are widely applied in practice.
Zinc phosphating processes based on low-zinc technology are also in use. Low-zinc technology is a variant which differs from normal zinc technology in certain significant aspects. These differences lie in particular in the concentrations in which the determining bath components, zinc and phosphate, are present in the treatment solution and in the molar and weight ratios of these two components to one another. Whereas in normal zinc phosphating baths the weight ratio of zinc to phosphate is approximately 1:(1-12), the weight ratio in low-zinc phosphating baths is approximately 1:(14-30).
German Patent Application No. 22 32 067 discloses that low-zinc technology in particular leads to phosphate coatings on metal which are superior to those obtained by normal zinc technology with regard to both lacquer adhesion and corrosion prevention. However, low-zinc phosphating processes are attended by disadvantages, above all regarding the management of the phosphating baths. The phosphating rate is lower in the low-zinc phosphating process, so that the throughputs are correspondingly lower. The bath components in the phosphating bath are consumed in a ratio to one another which differs significantly from the ratio in which they are present in the bath itself. Because of this, phosphating concentrates differing significantly in their composition are required according to U.S. Pat. No. 4,419,199 and corresponding European Patent Application No. 64,790, both for preparing and for replenishing the bath. In addition, phosphating baths are relatively difficult to monitor, especially since the ratio of chemical consumption to mechanical erosion, (which in turn depends among others upon the shape of the metal workpiece being treated, upon the drainage facilities and also upon the type of phosphating plant used), does not represent a constant value.